The present invention relates generally to energy sources useful in thermal processing apparatuses, more particularly to radiant heaters used in the rapid thermal processing of semiconductor workpieces or substrates, and especially to an apparatus and a method for heating semiconductor substrates during chemical vapor deposition (CVD) processes, including epitaxial reaction processes. Accordingly, it will be understood that when reference is made in the remainder of this application to CVD or epitaxial reaction processes, these are merely prime examples of the wider range of thermal processes to which the teachings of the present invention are applicable.
The commercial production of semiconductor devices has in recent times been placed under increasing pressure to reduce the cost per device. This in turn has required new measures to increase the efficiency of epitaxial processing methods so that they yield higher throughput of acceptable devices at a lower cost per device. Important developments in this regard are the use of radial heating arrays to improve heating efficiency and a compact, double-dome reactor which achieves increased processing speed and while reducing consumption of the gases used in semiconductor processing operations.
A representative double-dome apparatus is fully detailed in U.S. Pat. No. 5,108,792 entitled "Double Dome Reactor for Semiconductor Processing" which is commonly assigned with the present invention. The reactor system utilized in this cited patent is efficient enough to permit economic processing of even a single wafer per processing operation. The central concepts of this double dome reactor system may be summarized as follows: (1) support the substrate on a thin, low thermal mass susceptor for rapid heating and cooling; (2) enclose the substrate and susceptor in a compact housing having a pair of transparent domes, each dome covering one face of the susceptor-substrate combination; (3) provide conduits for reactant gases to enter and exit the housing; (4) provide a pair of radiant heaters arranged to project energy through each of the transparent domes to uniformly heat each face of the susceptor-substrate combination.
Representative radial heater arrays are fully detailed in U.S. Pat. No. 5,179,677 entitled "Apparatus and Method for Substrate Heating Utilizing Various Infrared Means to Achieve Uniform Intensity" which is commonly assigned with the present invention. The central concepts of the above cited patent is the reduction in heater complexity by utilizing: (1) a plurality of linear, tubular heater lamps; (2) providing focusing reflectors which cause thermal radiation to be directed in parallel paths towards the substrate resulting in greater thermal radiation near the center of the substrate or susceptor; (3) providing dispersive reflectors which cause thermal radiation to be dispersed in a hemispherical pattern and (4) providing a peripheral reflector circuferentially surrounding the lamps and their associated reflectors to intercept some of the dispersed radiation and redirect it to the periphery rather than at the center of the substrate or susceptor in order to balance the radiation pattern of the focusing reflectors.
While reactor systems utilizing the double dome reactor and radial heater arrays have proven highly efficient in reducing processing costs and increasing throughput, work has continued on further improvements driven by the semiconductor industry's trend toward increasing wafer or semiconductor workpiece diameters. Early wafers had diameters of 25 mm or less, while today the industry faces the migration from 200 mm wafers to 300 mm wafers. The trend toward larger wafer sizes is moderated by the added difficulty of obtaining the uniform heating required to maintain higher yields while utilizing ever increasing substrate diameters. The migration from 200 mm diameter to 300 mm diameter susceptors and substrates results in a larger surface area over which rapid, uniform thermal gradients and radiation patterns need to be employed. The semiconductor industry's previous substrate diameter increase from 150 mm to 200 mm--a surface area increase of 180%--necessitated improvements such as the double-dome reactor and radial arrays described in the aforementioned patents.
However, increasing the number and power of lamps used in radial arrays only partially solves the problem since the amount and degree of interference caused by interference between lamps in the various arrays increases proportional to the number of lamps utilized. The resulting interference results in heating inefficiencies which hinder thermal processing performance and diminish overall system throughput. What is needed to solve the difficulties posed by the 225% surface area increase presented by the migration to 300 mm diameter substrates and susceptors is a radiation directing device which effectively utilizes energy from multiple heater arrays. Such a device would thereby provide the higher energy necessary for high throughput thermal processing of existing and larger diameter substrates and susceptors.
These and other disadvantages of the prior art are overcome by the present invention directed to a method and an apparatus which can increase the efficiency of and further improve upon the uses for multiple energy sources to uniformly and rapidly heat existing and larger diameter susceptors and substrates. Such an apparatus increases the efficiency of radial heater arrays thereby reducing over system power consumption while maintaining rapid thermal processing capabilities.